This invention relates to a liquid crystal display device, and in particular, to a liquid crystal display device including an active matrix utilizing an MIM (metal-insulator-metal) element as a switching element.
A typical liquid crystal display device includes liquid crystal material between two substrates in which one substrate contains multiple picture elements. Each picture element is connected to a nonlinear element with two terminals. Typical nonlinear elements are those including inorganic films, such as in MIM elements, MSI (metal semi-insulator) elements, ring diodes, back-to-back diodes, varistors, and the like, or may include organic films such as polyimide. A typical nonlinear element is the MIM element which functions as a switching element to permit voltage retention. Each picture element is connected via its MIM element to its neighbor picture element by an interpixel connection.
A conventional liquid crystal display including a plurality of picture elements in a matrix on a substrate are each connected to an electrode by an MIM element as shown in FIGS. 1 and 2. A plurality of MIM elements 9 in a matrix on an insulating substrate 1 and the structure of MIM element 9 are shown. MIM element 9 includes a first metal film 2 on insulating substrate 1. First metal film 2 is used as a lead and as a terminal portion 3 for making contact with an external driving circuit and is formed with a plurality of projecting portions 4. An insulating film 6 is formed on the surface of metal film 2 except along terminal portion 3. A second metal film 7 is formed on insulating film 6 intersecting projecting portion 4 of first metal film 2. The intersection of first metal film projecting portion 4 and second metal film 7 with insulating film 6 therebetween creates a MIM element. A transparent picture cell electrode 8 is deposited on substrate 1 and connected electrically as a series with second metal film 7 of MIM element 9.
FIG. 3 shows the terminal area and first pixel of the type of device shown in FIGS. 1 and 2. FIG. 4 is a cross-sectional view taken along line 4--4 of FIG. 3. In this type of device, a picture element electrode 15 and MIM element 18 are located on the first position of interpixel connection 17. The structure shown in FIGS. 1 and 2 is formed as follows. A first metal film, typically tantalum (Ta), is formed on substrate 10 and photoetched to a desired shape such as parallel electrode lines 11. Ta first metal layer 11 is then oxidized by anodic or thermal oxidation to form a thin layer of tantalum oxide (TaO.sub.x) insulation layer 13, where x is any number which forms a stable oxide of Ta, overlapping Ta layer 11. A terminal area 16 is formed by removing the TaO.sub.x layer, leaving Ta layer 11 exposed at terminal area 16.
Interpixel connection 17 includes a projection 25 extending perpendicularly from Ta layer 11 at each pixel area. A second metal layer 14 of chromium (Cr) is formed and photoetched across projecting region 25 of Ta and TaO.sub.x. The intersection of Cr second metal layer 14, TaO.sub.x layer 13 and Ta layer 11 forms a non-linear element, or MIM element 18. Finally, a picture element electrode 15 is formed by depositing a transparent picture cell electrode material and photoetching to form a picture element electrode 15 overlapping end portions of Cr layer 14.
In the example shown in FIGS. 3 and 4, three intricate photoetching steps occur, namely the photoetching of Ta layer 11, Cr layer 14, and picture element electrode 15. Although the formation of terminal area 16 also involves photoetching, it does not require the accuracy to form the interpixel connection and is more easily accomplished than the other three photoetchings.
Signals reach picture element electrode 15 as follows. A signal is input to Ta layer 11 at terminal area 16 and travels along Ta layer 11 to interpixel connection 17. The signal reaches projection 25 and MIM element 18 and is then carried through TaO.sub.x film to Cr layer 14 and finally to picture element electrode 15. At the same time, the signal continues to travel along Ta layer 11 to reach additional picture elements and the respective MIM elements in the row.
A diagram of an equivalent circuit for the connection between terminal 16 and picture element electrode 15 is shown in FIG. 5. Distribution resistance (R) 21 indicates the distribution resistance of interpixel connection 17 which substantially equals the distribution resistance of Ta layer 11, since TaO.sub.x 13 is an insulator. MIM element 18 is the equivalent of capacitor C.sub.m and resistance r.sub.m connected in parallel and are indicated as numeral 19. Distribution resistance 21 and MIM 19 are connected in series with a picture element 20.
In view of this, the resistance of interpixel connection 17 depends only on the resistance of Ta layer 11, since TaO.sub.x layer 13 is an insulator. Since the resistance of interpixel connection 17 reduces the voltage of the signals input at terminal area 16, a voltage difference arises. Likewise, the voltage which reaches between a picture element close to terminal 16 and a picture element further away from terminal area 16 varies when the picture elements are displayed. In other words, unevenness of the display is caused. In order to improve picture quality of a liquid crystal display device of this type, it is necessary to reduce the resistance of interpixel connection 17.
Several methods have been suggested for reducing the resistance of interpixel connection 17 to improve the picture quality of the resulting liquid crystal display device. It has been suggested to enlarge the patterned width of Ta layer 11 at interpixel connection 17 or by increasing the thickness of Ta layer 11. The first suggestion is less than satisfactory, because the increased width of the interpixel connection reduces the density or picture elements which in turn reduces the picture quality. The latter suggestion not only adversely affects pattern accuracy at time of forming Ta layer 11, but also has a negative impact on forming the stepped chromium photoetching at the edges of the TaO.sub.x layer. Accordingly, it has been difficult to reduce the distribution resistance at the interpixel connection. This has led to propose a structure of interpixel connection 17 shown in FIGS. 6-8. The portion of the substrate of the liquid crystal display device shown in FIG. 6 corresponds to the same portion of the substrate in FIG. 3. This includes terminal area 16 and first picture element electrode 15 with interpixel connection 17. FIG. 7 is a cross-sectional view of line 7--7 of FIG. 3. FIG. 8 is a cross-sectional view along line 8--8 of FIG. 6. FIGS. 8a-8d show the steps of manufacturing the apparatus shown in FIG. 8e.
Ta layer 11 is formed and photoetched on substrate 10 and oxidized to form TaO.sub.x layer 13. A channel 23 is etched through the center of TaO.sub.x layer 13 to expose a portion of Ta layer 11. Channel 23 is filled with a second metal layer, such as Cr layer 12 which is coated and photoetched at the same time as Cr layer 14. After this, picture element 15 can also be formed and photoetched as in the earlier examples.
The resistivity of chromium is approximately 30 micro-ohm-cm, much less than the approximately 220 micro-ohm-cm of tantalum. Thus, the resistivity of interpixel connection 17 which includes both Ta layer 11 and Cr layer 12 is considerably less than the resistivity of an interpixel connection without Cr layer 12. Additionally, the thickness of Cr layer 12 may be increased as desired for optimum consistency in the voltage applied to the different picture elements along Ta layer electrode 11.
A liquid crystal display device formed from a substrate as shown in FIGS. 6-8 has two disadvantages. First, the additional photoetching step shown in FIG. 8c, as well as the photoetching of Cr layer 12, increases the number of photoetching steps to four, and thus the production cost by at least one-third. Further, during the additional photoetching step of FIG. 8c, a portion of TaO.sub.x layer 13 of MIM element 18 is exposed to the photoetching process which results in a higher incidence of imperfections in the insulation layer of the MIM device and thus a decreased yield of substrates during production.
Accordingly, it is desirable to provide an active substrate for a liquid crystal display device capable of improved contrast between picture elements by lowering the resistivity of interpixel connections without an increase of production costs or reduced yields.